This invention relates to pressure transducers.
In a key respect, the invention relates to melt pressure measurement of plastically deformable materials, for example, synthetic resins, pastes, slurries, and food substances. Plastic materials such as these are melted or mixed and then extracted or injection molded to form solid objects such as injection molded parts, extruded shapes, fibers, extruded film, and shaped food stuffs. During the process, the material is typically heated to a desired temperature, which may range as high as 1000.degree. F., and worked under a desired pressure, which may range as high as 20,000 or 30,000 psi or higher.
It is important to accurately measure the pressure of the material while it is processed in both manufacturing and laboratory settings. In rheological studies, for example, the mixing, flow, viscosity, and separation properties of the plastic material being extruded through a vessel are all detected and analyzed as a function of the melt pressure of the material within the vessel. In manufacturing applications, the melt pressure of the material being extruded or injection molded is monitored for quality control of the process parameters as well as to guard against dangerous conditions resulting from equipment malfunction.
Referring to FIG. 1a, as currently practiced in industry, a conventional melt pressure transducer 10 is typically installed in one or more apertures 11 located along the length of an extruder barrel 12 to measure the melt pressure of the molten material as it flows past the transducer and toward, for example, an extrusion die 14. As shown in FIG. 1b, a conventional melt pressure transducer 10 consists of a hollow, elongated rod-shaped housing having a flat diaphragm 16 located at the end of the rod tip 18. Threads 20 along the outside of the housing mate with corresponding threads in the extruder barrel aperture 11 to mechanically fix the transducer in the aperture such that the tip diaphragm 16 is exposed to the melt stream in the barrel bore. A high pressure seal 15 is formed between the high pressure chamber and the threads so that the melted plastic will not reach the threads, and the probe is sufficiently long to extend through the thickness of the barrel, e.g., 2 inches or 4 inches, and through barrel band heaters and thermal insulation.
Pressure exerted on the tip diaphragm 16 by the melt stream is transferred outwardly from the melt to a second, sensing diaphragm 22 within the housing by way of a push rod or a liquid-filled capillary tube within the housing (both shown generically as 24). Mercury is typically used as the capillary liquid, except for temperatures above its boiling point, where a sodium-potassium mixture may be used. The melt pressure applied to the tip diaphragm 16 is transferred to the fill liquid which exerts this pressure on the sensing diaphragm 22 via the capillary tube 24. A strain gauge bridge (not shown) mounted on the second diaphragm 22 senses diaphragm strain, caused by the pressurized liquid, which results in bridge resistance changes. Circuitry (not shown) interfaces with the strain gauge bridge to generate a signal indicative of the melt pressure. Pressure on the tip diaphragm 16 is transferred to a remote diaphragm 22 for sensing and measuring because the high temperature prohibits reliable bonding to and operation of the strain gauges.
The use of either a push rod or a liquid-filled capillary to transmit pressure from the tip diaphragm 16 to a second remote diaphragm 22 significantly limits the performance of conventional melt pressure transducers. Typically, the diaphragms, push rod (if used), and housing are all of thermally conductive materials, which differentially expand or contract in response to changes in temperature. Slight differences in thermal expansion coefficients or thermal gradients can cause a differential expansion between the housing and push rod which results in the application of pressure (in addition to the melt pressure) on the sensing diaphragm, thereby distorting the transducer's pressure measurement. Due to this temperature sensitivity, push rod transducers are quite thermally unstable and hence limited in their accuracy and dependability.
Liquid-filled pressure transducers, while exhibiting better temperature stability than push rod transducers, pose serious safety hazards; abrasion of the transducer tip diaphragm or cyclic fatigue can cause leakage of the liquid, typically highly toxic Mercury, into the melt. In addition, the boiling point of the liquid sets the upper boundary of melt temperature to which the transducer may be exposed; for Mercury, the upper temperature limit is about 750.degree. F. Sodium-potassium, while capable of use at higher temperatures, is a spontaneously combustible substance which presents particular hazards in manufacture of the transducer, as well as in use.
Finally, both push rod and liquid-filled transducer designs place an inherent limitation on the frequency response of the transducers. The useful frequency bandwidth is inversely proportional to the length of the push rod or capillary tube; the longer the rod or capillary, the more severely the useful frequency range is restricted This restriction limits, e.g., the scope of rheological data that may be gathered using the transducer. For such reasons there has been a need for better means for melt pressure measurement.